A prospective study in which the effect of the post-ictal systemic blood pressure levels on the outcome following surgically treated aneurysmal subarachnoid haemorrhage (SAH) in 325 patients is reported. No differentiation was made between pre-existing essential and prolonged reactive hypertension. A significant association was found between the trend for rising diastolic blood pressure levels and the tendency towards a poor outcome. There was a similar trend for rising systolic blood pressure levels which did not reach statistical significance. Patients with definite hypertension (blood pressure of 160/95 mmHg or greater) were 1.6 times as likely to have a poor outcome than were those with lower blood pressures.